The present invention relates to a method for controlling and adjusting an ink density during the printing process in rotary printing machines for offset printing. More particularly, the invention relates to a method for controlling ink density in rotary roller printing machines.
Optimal ratios of the amounts of printing ink and moisturizing agent fed to the plate cylinder are important to obtain good quality of the offset printing. It is not sufficient to maintain the ratio of the aforesaid amounts within predetermined allowance limits, between which ink or water smearing marks in the printing would not occur.
It has been common practice to believe that the quality offset printing depends on the determined moisture agent portion in the printing ink. The elasticity of the printing ink depends on the portion of the moisture agent emulsified in the printing ink so that the full tone or shade density and the tone or shade value of the printing are considerably influenced.
The emulsifying of the moisture agent in the printing ink is caused by high pressure at contacts between the ink applying rollers and the plate cylinder. The portion of the moisture agent in the ink-moisture agent emulsion depends upon the moisture agent supply and the moisture agent need for the image-free locations of the printing plate--, upon the moisture agent amounts available on the printing plate and the properties of the moisture agent and the printing ink, the emulsifying quality of the ink-moisture agent mixture, the peripheral speed of the plate cylinder, rubber cloth and printing material, environmental temperatures and humidity and other various influencing factors.
With widely utilized printing machines with a moisture agent feeding provided over the printing machines width it has been obtained that various zonal requirements of the moisture agent corresponding to the ink profile for the upper surface of the image-free locations have been compensated for by the moisture agent content in the ink-moisture agent emulsion. The zonally-different ink-moisture agent emulsion is formed automatically. It has been disclosed, for example in DE-OS 29 31 579, DE-OS 32 11 157 and DE-OS 32 20 701 that the quality offset printing requires as uniform and optimal ink-moisture agent-emulsion over the entire width of the printing machine as possible, which leads to required profiles and the zonal moisture agent supply.
In the DE-OS 29 31 579 the control characteristic curve of the zonally fed moisture agent amounts is controlled according to zonally fed ink amounts without an answer-back signal indicative of the emulsified moisture agent amounts. The disadvantage of this otherwise satisfactory method resides in that it does not react to local flucuations of the ink-moisture agent ratios caused by such disturbing factors an environment temperatures and humidity and machine heating. A further disadvantage of this known method is that no correction possibilities are provided for various emulsifying properties of different ink-moisture agent mixtures or various moisture agent storing abilities of the printing plate.
The method disclosed in DE-OS 32 11 157 teaches the step of the zone-wise measurements of the moisture agent amounts on the upper surface of the printing plate and computing of the control deviation and place of contact. The emulsified moisture agent amount is however not determined with this solution.
In DE-OS 32 20 701, a hydrophilic measuring roller operates in contact with the ink applying rollers and a measuring device which issues a signal indicative of wetting of the measuring roller with ink or moisture agent over the printing machine width in portions. As soon as the portion of ink increases on the measuring roller at which the moisture agent supply is near the lower tolerance limit an additional moistening takes place in this portion, and as soon the portion of ink increases at the measuring roller at which the moisture agent supply is near the upper tolerance limit, the moisture agent is wiped off in this portion. The disadvantage of this conventional solution resides in that in connection with rigid control of the moisture agent supply the moisture agent feeding principally works in the proximity of one or the other tolerance limit, and the optimal ink-moisture agent radio of the emulsion is excluded.
Also known are a method and device for a separate measurement of the amount of the moisture agent on the upper surface of the roller and the emulsified moisture agent amount. These features have been disclosed in DE-OS 34 44 784. The measurement takes place in a contactless fashion and is executed by a diffused light reflection through the emulsified moisture agent. The separate measurement enables the influence of the ink-moisture agent emulsion.
The disadvantage of the last mentioned method resides in high costs of the equipment. The light intensity measurement of the reflected light beam as well as of the directed and also diffused beam portion and the intensity measurement of the reflected diffused beam portion require photo-electronic transmission means and A/D conversion means for signals measured and as well as a number of evaluation means to determine the amounts of the moisture agent. A particular disadvantage of the solution resides in the necessity to provide calibration of the measuring device. A high expensive is connected also with a quick insertion.